This invention relates generally to an apparatus and method for testing alternating current (AC) motor power systems and, more particularly, to an apparatus and method for simultaneous non-destructive in-circuit testing for short-circuit conditions of multiple inverters in an alternating current system.
It is common in AC electric motor drive systems to employ power inverter systems to furnish electric power from a DC source to an AC motor. These inverter systems are of various types, but are often comprised of gate turn-off thyristors (GTO) in a bridge arrangement. The GTOs in the bridge are selectively gated to control the electrical power supplied to the motor by converting DC power from the DC source into AC power which drives the motor. Typically, two GTOs are connected in a series arrangement in what is commonly referred to as a "leg" or "phase" between relatively positive and relatively negative busses of the DC source. A common inverter of this type is a three-phase inverter having three legs connected in parallel between the positive and negative DC source busses. The GTOs of each of the legs are rendered conductive in a predetermined order or sequence in order to control the electrical power delivered from the DC busses to the AC motor. In an exemplary application of such inverters such as, for example, in a locomotive electric drive system in which a diesel engine drives an alternator at variable speed to produce variable frequency electric power which is rectified before inversion to a controlled frequency power, the electric drive system can be reconfigured to use the inverter to drive the alternator for starting the diesel engine. This configuration is generally referred to as a "cranker" mode and uses the locomotive battery (typically about 75 volts) as the power source.
The above described motor systems require regular maintenance to avoid or detect a variety of common failure modes. For example, if both GTOs of a leg were to become conductive simultaneously, there would exist a short between the DC source busses which, if allowed to continue, could result in great damage to the motor, power source, and/or to the GTOs. In addition, failure of various components and segments of the system such as the feedback circuits, drive circuits, and the motor load itself can occur. U.S. Pat. No. 5,363,039, assigned to the assignee of the present invention, discloses a method and apparatus for auto self test of an inverter in an AC motor drive system. However, that patent does not address how to simultaneously test a plurality of inverters in a system, such as a locomotive, in which four, six or eight inverters may each be coupled to supply AC power to a corresponding AC motor driving a wheel-axle set of the locomotive. While the system of the '039 patent can self test each inverter, it is necessary to do so in a sequential format. With six inverters, 36 separate tests must be performed resulting in noticeable delay in bringing the locomotive to an operating condition. Accordingly, it is highly desirable to provide an auto self test capability which can simultaneously test each inverter in a multiple inverter system to identify short-circuit conditions in any of the inverters. In U.S. Pat. No. 5,363,039 the shorted module detection requires that the DC link be charged rapidly to a relatively high voltage. In a locomotive where the power circuit can only be charged slowly from an alternator or to a low level from a battery, that system is not satisfactory.